The Human Immunodeficiency Virus (HIV) infects millions of people globally. Cases are reported from nearly every country amounting to 40 million adults and children living with HIV/AIDS worldwide. In 2001, 5 million people were newly infected with HIV, and there were 3 million adult and child deaths due to HIV/AIDS. A full third of those people living with AIDS are aged 15-24. (World Health Organization, 2001). HIV/AIDS treatments exist, however, the drugs currently used in treatment modalities exhibit numerous side effects, require prolonged treatment that often induces drug resistance, and do not result in complete eradication of the virus from the body.
The disease AIDS is the end result of an HIV-1 or HIV-2 virus following its own complex life cycle. The virion life cycle begins with the virion attaching itself to the host human T-4 lymphocyte immune cell through the bonding of a glycoprotein on the surface of the virion's protective coat with the CD4 glycoprotein on the lymphocyte cell. Once attached, the virion sheds its glycoprotein coat, penetrates into the membrane of the host cell, and uncoats its RNA. The virion enzyme, reverse transcriptase, directs the process of transcribing the RNA into single-stranded DNA. The viral RNA is degraded and a second DNA strand is created. The now double-stranded DNA is integrated into the human cell's genes and those genes are used for virus reproduction.
At this point, RNA polymerase transcribes the integrated DNA into viral RNA. The viral RNA is translated into the precursor gag-pol fusion polyprotein. The polyprotein is then cleaved by the HIV protease to yield the mature viral proteins. Thus, HIV protease is responsible for regulating a cascade of cleavage events that lead to the virus particle's maturing into a virus that is capable of full infectivity.
The typical human immune system response, killing the invading virion, is taxed because the virus infects and kills the immune system's T cells. In addition, viral reverse transcriptase, the enzyme used in making a new virion particle, is not very specific, and causes transcription mistakes that result in continually changed glycoproteins on the surface of the viral protective coat. This lack of specificity decreases the immune system's effectiveness because antibodies specifically produced against one glycoprotein may be useless against another, hence reducing the number of antibodies available to fight the virus. The virus continues to reproduce while the immune response system continues to weaken. Eventually, the HIV largely holds free reign over the body's immune system, allowing opportunistic infections to set in and, without the administration of antiviral agents, immunomodulators, or both, death may result.
There are at least three critical points in the virus's life cycle which have been identified as possible targets for antiviral drugs: (1) the initial attachment of the virion to the T-4 lymphocyte or macrophage site; (2) the transcription of viral RNA to viral DNA (reverse transcriptase, RT); and (3) the processing of gag-pol protein by HIV protease.
Inhibition of the virus at the second critical point, the viral RNA to viral DNA transcription process, has provided a number of the current therapies used in treading AIDS. This transcription must occur for the virion to reproduce because the virion's genes are encoded in RNA and the host cell reads only DNA. By introducing drugs that block the reverse transcriptase from completing the formation of viral DNA, HIV-1, replication can be stopped.
A number of compounds that interfere with viral replication have been developed to treat AIDS. For example, nucleoside analogs, such as 3′-azido-3′-deoxythymidine (AZT), 2′,3′-dideoxycytidine (ddC), 2′,3′-dideoxythymidinene (d4T), 2′,3′-dideoxyinosine (ddI), and 2′,3′-dideoxy-3′-thia-cytidine (3TC) have been shown to be relatively effective in halting HIV replication at the reverse transcriptase (RT) stage.
Even with the current success of reverse transcriptase inhibitors, it has been found that HIV patients can become resistant to a single inhibitor. Thus, it is desirable to develop additional inhibitors to further combat HIV infection and inhibit the replication of drug resistant strains of HIV.